The present invention generally relates to information recording/reproducing technologies capable of optically recording and reproducing information to and from an information recording medium (optical disk) from which the information can be read optically. More specifically, this invention relates to a method suitable for an optical disk so formatted as to correspond to high density information recording, and to an information recording/reproducing apparatus achieving the functions of this method.
In optical disks in general, the information is reproduced by condensing a laser beam to an information recording surface of the optical disk and detecting those reflected beams which are modulated by recording marks and pits. The recording marks are formed along a guide groove on the spiral formed on the information recording surface. As described in JP-B-4-4661, the width of a groove and the width of a portion between the grooves are arranged to be substantially equal and the depth of the groove is set to the depth at which cross-talk from adjacent tracks determined by the wavelength of the laser beam to be irradiated becomes minimal. In consequence, the recording marks can be formed similarly on the guide grooves (groove tracks) and on the portions between the grooves (land tracks), reproduction of the information can be made, and the recording density in the radial direction of the disk can be improved.
When the groove is formed simply and continuously from the inner circumference to the outer circumference and when the reproducing operation shifts from the continuous reproduction on the groove track to the continuous reproduction of the land track, the time involved with this shift is necessary in order to continuously reproduce the information recorded on the groove track and on the land track on the optical disk, and continuous reproduction of the information on the land and groove tracks becomes impossible.
Therefore, as shown in JP-A-7-141701, if a groove (single spiral) is formed in such a fashion that the land track and the groove track appear alternately and substantially in each circumference, the information recorded on the land track and on the groove track can be reproduced continuously.
To reproduce the recording marks formed on the land track or the groove track, the condensing position of the laser beam on the information recording surface must be scanned along the substantial center of the land track or the groove track in the radial direction of the optical disk. The condensing position of the laser beam at this time can be identified from deviation (tracking deviation amount) which is substantially proportional to the distance from the substantial center position of the land track or the groove track by using diffraction components of the laser beam by the groove. The condensing position of the laser beam is controlled (tracking control) by using this deviation amount but the polarity of the deviation reverses between the land track and the groove track. In order to carry out continuous reproduction from the land track to the groove track or vice versa, therefore, the condensing position of the laser beam must be controlled in such a fashion as to correspond to the reversion of the polarity of the deviation amount at the alternating position of the land track and the groove track.
Therefore, regions are set in the radial direction substantially equidistantly in the circumferential direction on the track of the optical disk and pits are formed inside these regions, as described in JP-A-6-176404. In this instance, the pits are formed so that when the information of the pits is reproduced, the alternating position (polarity reversing position) between the land track and the groove track can be identified, and the tracking control state is changed over on the basis of this information.
The pits are formed in advance (or "preformatted") substantially equidistantly in the circumferential direction of the track. When these pre-pits are so formed as to alternately offset mutually at positions spaced apart by a quarter of the distance of the center position of each groove track adjacent to the land track in both inner and outer circumferential directions from the center position of the land track or the groove track, the signal that reproduces the bit information on the deviation signal representing the tracking deviation amount can be outputted.
In other words, it is possible to identify the formation position of the pre-pit (PID: Physical Identification portion) by detecting the position where the tracking deviation amount alternates to the positive and the negative in the cycle of the pit information and also to judge which of the land track and the groove track the optical spot scans at present from the positive and negative sequence of the signal output corresponding to the pits.
FIG. 2 is a schematic block diagram of an optical disk apparatus including a PID position detector. An optical disk medium 1 is rotated by a spindle motor 2 substantially at a constant linear velocity. An optical head 3 includes optical components for condensing for a laser beam on an information recording surface of the optical disk, a detector for detecting a reflected beam, a PD track head for making the condensing position variable, and so forth.
Automatic gain controllers 1 (AGCs) 11 and 12 control an RF (Radio Frequency) signal as the level change of the reflected beam from the optical head and an HPP (High-Frequency Push-Pull) signal corresponding to the tracking deviation amount to constant amplitude levels. The amplitude of the HPP signal changes with variance of the groove depth of the optical disk, with variance of the reflection factor of the optical disk, with variance of the laser beam quantity irradiated to the optical disk and with sensitivity variance of a detector which receives the reflected beam from the optical disk and converts it to an electric signal. Further, the waveform amplitudes vary between the land track and the groove track when the laser beam is so condensed at the PID portion as to deviate from the center position of the track or when the laser beam turns to an elliptic spot at the condensing position while deviating from the vertical axis of the optical disk.
For this reason, PID position detection cannot be carried out stably at the slice level of the comparator described above as a fixed level against the variation of the HPP signal amplitude, and PID detection is effected after the amplitude is controlled to a predetermined amplitude by the automatic gain controllers (AGCs). Because the signal amplitude of the RF signal changes with the recording condition, the amplitude is rendered constant by the AGC so that digitization can be carried out stably.
A switch 13 switches the PID portion and the data track on the optical disk by a signal switch signal 32 from the PID position detector, selects at the PID portion the output of the AGC 13 that has processed the HPP signal and selects on the data track the output of the AGC 12 that has processed the RF signal. The output of this switch 13 is inputted to a DC corrector 14 to render a DC level of each signal substantially constant. For example, envelopes at the upper and lower portions of each signal are detected in a reference signal region at the leading part of each signal and control is made so that the mean level of the upper and lower envelopes attains an arbitrary level.
The output of the DC corrector 14 is inputted to an automatic slice level controller (ASC) 15 and is digitized at a suitable slice level. The output of this ASC 15 is inputted to a phase-locked loop (PLL) 16 and a reproduction clock synchronized with the digitized signal is generated. The reproduction clock is inputted with the digitized signal to a reproduction signal demodulator 17 so that recording information is demodulated from the RF signal of the data track while the position information of the optical disk is demodulated from the HPP signal of the PID portion. When this HPP signal is demodulated, the information about alternation of the land track and the groove track can be obtained, too.
The pit information of the PID position contains the information about the position on the optical disk and the information as the reference for the reproduction processing, and the position information is formed in succession to the reference information. Generally, the signal frequency of the reference information is constant and the reproduction processing system executes control by using this reference information, so that the position information can be stably reproduced. Incidentally, the RF signal, too, contains the reference information at the leading part thereof in the same way as the PID portion, and the recording information is formed in succession to this reference information.